FIG. 1 illustrates a prior art electric power generating system (EPGS) 7 of the type manufactured by the assignee of the present invention for use in airframes. The EPGS 7 is comprised of a plurality of generating channels 37. Each channel comprises a generator unit such as an integrated drive generator 9 coupled to an input shaft (not illustrated) from an airframe propulsion engine. The output of the IDG 9 is connectable by a generator control breaker (GCB) 31 to a load distribution bus 23 which is connectable by a bus tie breaker (BTB) 29 to a tie bus 35. Position control for each breaker is transmitted via control line 33 to the GCB 31, and via control line 27 to the BTB 29. Each IDG 9 is conventional and is comprised of a constant speed transmission and a permanent magnet generator which generates alternating current which is rectified and applied, through a series connected generator control relay (GCR) (not illustrated), to a wound field exciter on line 19 which produces alternating current which is rectified and applied to the rotor of a three phase alternator. The number of IDGs 9 included in the EPGS 7 varies directly with the number of engines on the airframe and typically is between 2 and 4. The rotor of the three phase alternator is driven by the constant speed transmission (included within the IDG 9) which converts a variable speed shaft from the airframe propulsion engine into a constant speed drive which rotates the rotor of the three phase alternator at a velocity for producing three phase 400 Hz electrical power. Each IDG 9 has an associated generator control unit (GCU) 11 which may contain a programmed microprocessor or other means for implementing various conventional control and protection functions as well as functions which are described below which are pan of the present invention.
In addition to the main engine driven IDGs 9, an auxiliary power unit driven generator unit (AGEN) 13 is often included as an integral pan of the EPGS 7. The AGEN 13 is connectable by an auxiliary power breaker (APB) 39 to the tie bus 35 to allow the AGEN 13 to power the main channel load busses 23 via the BTBs 29 during IDG fault or loss of engine conditions, or while on the ground without main engines running. As with the IDGs 9, the AGEN 13 has an associated generator control unit (AGCU) 15 which also may contain a programmed microprocessor or other means for implementing various conventional control and protection functions. Often, the GCU 11 and the AGCU 15 are identical units, differing only in the control algorithms executed by the microprocessor. Also included is a connection 17 to allow external power (EXT PWR) to be connected to the tie bus 35 to supply the main channel's load distribution busses 23 through the BTBs 29 while on the ground.
To protect the utilization equipment from a low voltage operating condition, the GCU 11 utilizes generator speed information, and voltage sense line 41 to monitor system voltage at the point of regulation (POR) 43. The GCU 11 processes the information collected by these sensors via a voltage level detector 47 (see FIG. 2), which calculates an average voltage magnitude and generates an under voltage control signal when the average voltage magnitude drops below a threshold, and via a generator speed monitoring means 49 which generates a ready speed control signal when the generator speed is above a minimum regulation speed.
The under voltage protection logic 45 as shown in FIG. 2 monitors these control signals and the position status of the GCR (not illustrated) via a GCR monitor 51 to determine if an under voltage fault condition exists. Specifically, logic gate 53 generates an under voltage protection signal on line 55 when a logic "1" is produced by the GCR monitor 51 on line 57 indicating that the GCR is closed allowing excitation power to be connected to the wound exciter field of the IDG (not shown), and a logic "1" is produced by the speed monitoring means 49 on line 59 indicating that the IDG is being driven above its minimum regulation speed, and a logic "1" is produced by the voltage level detector 47 on line 61 indicating that the average voltage on the POR 43 (FIG. 1) is less than a predetermined threshold ( this threshold is set to 104.5.+-.1.5 volts for a 115 volt aircraft EPGS). Once the under voltage protection signal on line 55 is generated, time delay 63 begins to operate. The duration of the time delay 63 is predetermined to coordinate with the trip characteristics of aircraft distribution protective devices (not shown), and is typically set to 9.75 seconds. A time delay duration of 9.75 seconds allows faults downstream of the distribution protection devices which overload the IDG and result in an under voltage condition to be cleared by these devices prior to generating an output protection signal on line 65.
If the undervoltage fault persists beyond the time by which the distribution protective device should have tripped to clear the fault, indicating a non-distribution type fault, the time delay 63 expires and the output under voltage protection signal on line 65 is generated. In response to this protection signal, the GCU 11 (see FIG. 1) trips the GCR (not shown) and GCB 31 which de-energizes the IDG 9 and disconnects it from the load distribution bus 23. The GCU 11 then may close the BTB 29 to allow an alternate source to re-power the loads via the tie bus 35.
One problem associated with this protection system is that for many non-distribution type faults resulting in an under voltage condition, such as failures within the IDG 9 or the GCU 11 or on the exciter drive line 19 itself, the utilization equipment is subjected to a low voltage for the duration of the time delay 63 (see FIG. 2) which is set to coordinate with distribution protection devices which would never operate to clear these types of faults. If, for example, the exciter drive line 19 were to break open, disconnecting the excitation power from the wound exciter (not shown), the output voltage would drop to zero for 9.75 seconds. All of the utilization equipment connected to the affected load distribution bus 23 would be lost during this period until the GCU 11 operates to disconnect the de-energized IDG 9 from, and connect the tie bus 35 to, the load distribution bus 23. If this fault were to occur during a critical phase of the flight, such as on take-off or landing, the loss of essential flight instruments and controls for this long period of time could result in serious consequences.
Another problem associated with this protection system for under voltage faults resulting from a controller or control wiring failure is that much of the utilization equipment utilize volatile memory for much of their data storage, the contents of which are lost as a result of a 9.75 seconds loss of power. This loss of data can be prevented by increasing the size of storage elements in the utilization equipment's power supplies or other techniques such as the use of non-volatile memory. Both of these solutions, however, increase the cost and the complexity of each piece of utilization equipment.
The present invention is directed to overcoming one or more of the above problems by discriminating the cause of the under voltage condition between that which is the result of an overload or a through fault on a main load bus, requiring coordination with aircraft distribution protective devices and thus a long time delay, and that which is the result of a controller or control wiring failure, requiring no coordination with the aircraft distribution protective devices and thus a very short time delay.